In their crude state, many gases used today as sources of energy contain sulfur compounds, in particular hydrogen sulfide. This is also true of sludge gas. Sludge gas, which is often also called sewer gas, is evolved in the anaerobic treatment of sludge. In closed towers, sludge is treated with microbes to convert it into a form in which it is environmentally harmless. In this process, up to 25 cubic meters of sludge gas or sewer gas--that is, a methane gas having a calorific value of nearly 23,000 KJ/m.sup.3, is produced per cubic meter of raw sludge. This methane gas is highly suitable for use in gas motors, to generate power from heat in sewage treatment plants, and for being combusted in gas burners used in heating sewage treatment towers and buildings, and after suitable processing it can also be fed into the public utility gas supply network. In each of these applications, however, using gas that contains sulfur is problematic. For instance, if the gases are combusted in internal combustion engines (ICE's) or gas burners, highly corrosive sulfur compounds such as H.sub.2 SO.sub.3 or H.sub.2 SO.sub.4 are produced. Accordingly, the necessary equipment must be made of corrosion-resistant materials, or else considerable repair and maintenance costs due to corrosion damage must be expected. On environmental protection grounds, it is furthermore undesirable, or even illegal, to emit sulfur-containing exhaust gases into the atmosphere.
To avoid these problems, the gases, such as the sludge gas produced in sewage treatment plants, are typically desulfurized in gas desulfurization plants. Such plants operate by either what is known as the dry process or what is known as the wet process. In the dry process, the hydrogen sulfide is converted with a solid cleaning composition, such as ferric hydroxide or Fe(OH).sub.3, into FeS and H.sub.2 O, after which the cleaning composition can be regenerated by oxidation. The sulfur is removed along with a portion of the cleaning composition and disposed of at special waste disposal sites. The used cleaning composition must be replaced.
In the wet process, the gas that is to be cleaned is washed with a liquid, and soluble sulfur compounds are formed in the liquid by chemical reaction. The liquid is then aerated with air or pure oxygen to oxidize the sulfur compounds. Depending on the chemicals used in the washing solution, gypsum (CaSO.sub.4) or ammonium sulfate ((NH.sub.4).sub.2 SO.sub.4), for instance, is formed, or the sulfur is precipitated out in a virtually chemically pure state. The liquid from which the sulfur has been removed can then be used again as washing liquid for the gas that is to be cleaned. The washing liquid as a rule contains additives that catalytically enable specifically filtering out the sulfur or its chemical compounds.
One wet process that is already known is called the Takahax process. In this process, the hydrogen sulfide is removed from the crude gas in a vertical container by spraying an alkaline liquid from the upper end of the container, through which the gas flows in a countercurrent to the falling droplets. The subsequent oxidation of the sulfur compounds is effected in tall upright cylindrical containers. These containers are open at the top and are filled with the more or less saturated liquid from the washing step. Compressed air is blown in large bubbles into this liquid through perforated pipes located on the bottom of the container. Oxidation occurs as the air bubbles rise to the surface of the liquid. The speed of the reaction is known to be dependent not only on the catalyst-specific energy potential and on the temperature, among other factors, but also substantially on the size of the contact surfaces between the air bubbles and the liquid. During the oxidation, elemental sulfur is liberated, some of which appears in the form of foam in an air filter above the oxidation vessel, where it is removed by a removal device. The sulfur is then filtered out in a filter, and the resultant liquid can be returned to the washing step along with the other regenerated liquid. (See the above-referenced article by Hasebe in the journal gwf: Das Gas- und Wasserfach.)
The great disadvantage of the Takahax process is that it requires complex and expensive apparatus and involves high capital investment and energy costs. For example, a Takahax plant was built in 1978 for a waste water treatment plant near Tokyo; it reduces the hydrogen sulfide content in 150 m.sup.3 /h of sludge gas from 0.5% by volume to about 0.05% by volume. This plant requires a washing tower 1.7 m in diameter and 13 m high, as well as an oxidation tower 1.3 m in diameter and 9 m high. Accordingly, the volume of the plant is approximately 42 m.sup.3, and approximately 15 m.sup.3 of washing liquid are required for the initial filling of the system.
In the Takahax process, an alkaline solution is used as the washing liquid, normally in combination with sodium 1,4-naphthoquinone-2-sulfonate as the catalyst.